Potentiometric sensors measure potentials where resistances are large, as is the case with pH-sensors and redox-sensors. PH-electrodes, respectively redox-electrodes, register particularly ionic potentials in solutions. In many applications, they are exposed to heavy wear, such that they are often replaced after short operating times. In this respect, these electrodes are consumable materials, which are to be provided as cost-effectively as possible with given precision of measurement.
The present invention will be explained on the basis of examples of pH-electrodes, respectively pH-sensors. However, the embodiments logically apply to other potentiometric sensors as well, particularly redox-sensors, respectively redox-electrodes.
Essentially three types of pH-sensors are known in the state of the art, and these will now be outlined briefly.
The simplest pH-sensors are simple pH-electrodes without any electronics. These pH-electrodes deliver a pH-dependent potential, which can be accessed at suitable electrical connections. Optionally, these pH-electrodes have an integrated temperature sensor for temperature compensation, e.g. PT100, the potential of which can be measured at suitable temperature outputs. For measuring, these pH-sensors are normally connected by means of a cable to a transmitter, which generates a measuring signal from the pH-dependent potential and, if necessary, from the temperature signal of the temperature sensor.
In addition to the described, simple pH-electrodes, respectively sensors, described, there are such with an integrated preamplifier for impedance conversion. The output signal of the preamplifier is the potential of the pH-sensor, with the internal resistance of the preamplifier amounting to only a few ohms, instead of the internal resistance of the pH-sensor in the order of magnitude of 100 megohms. Thus, the further transfer to, and processing of the output potential for, a transmitter is greatly simplified. The preamplifiers are either battery-powered, or are fed by means of a cable.
Finally, simple transmitters, which are mounted directly on the pH-sensor, are available under the name “DirectLine” from Honeywell. Consequently, in the direct vicinity of the sensor, e.g. a 4-20 mA measurement signal is generated, which can then be transferred without further ado to a control room.
For all pH-electrodes, respectively pH-sensors of the state of the art, it is necessary to calibrate the electrodes after the connection to the transmitter, whereupon the acquired calibration parameters are stored in the transmitter. Although this way of doing things enables, in most cases, satisfactory measuring operation following the calibration, it does have, however, several serious disadvantages.
Sensors must be recalibrated when they are connected to another transmitter. Sensors without prior calibration are not operable.
Sensor-specific additional information, such as type designation, service life, or historical data, is not available for every sensor, or is available only with great effort, especially when the affiliation a sensor and a transmitter has been lost.
Sensors must normally be calibrated at the site of the transmitter. Particularly in cases where adverse working conditions prevail at the site of transmitter, complex calibrations, such as the identification of the isothermal working point, are practically impossible to perform. This leads to compromises with respect to the achievable precision of measurement.